1. Field of the Invention
The present invention relates to wireless telecommunications and, more particularly, to a method and system that uses overlapping frequency bands in a hybrid frequency reuse plan for a wireless telecommunications network.
2. Description of Related Art
There has been an increased interest in providing wireless telecommunications networks that support high rate packet data communications. In the area of spread spectrum wireless communications, EVDO (Evolution Data Optimized) has been developed as a way of providing high speed data communications in cdma2000 networks. In the EVDO approach, a combination of code division multiple access (CDMA) and time division multiple access (TDMA) is used for downlink communications, i.e., communications from the base station to the mobile station (the forward link), and CDMA is used for uplink communications, i.e., communications from the mobile station to the base station (the reverse link). Different modulation schemes can be used for downlink and uplink communications, depending on signal-to-noise ratios. In this way, higher signal-to-noise ratios can support modulation schemes that support higher data rates.
As described in the original specification, EVDO was a frequency division duplex (FDD) approach, with one 1.25 MHz frequency band (e.g., a CDMA frequency channel) used for downlink communications and a separate 1.25 MHz frequency band (e.g., another CDMA frequency channel) used for uplink communications. Moreover, these frequency bands were reused in a K=1 frequency reuse plan. Thus, the same 1.25 MHz downlink frequency band and the same 1.25 MHz uplink frequency band were reused in adjacent cells and sectors through the use of different pseudonoise (PN) code offsets. The original EVDO approach could support a peak downlink data rate of 2.4 Mbps and a peak uplink data rate of 153.6 kbps. Revision A of EVDO enabled even higher downlink and uplink data rates.
Revision B of EVDO, however, includes some significant changes in the area of frequency usage. Revision B is described in 3rd Generation Partnership Project 2, “cdma2000 High Rate Packet Data Air Interface Specification,” 3GPP2 C.S0024-B, v1.0 (May 2006), which is incorporated herein by reference. As one significant change, Revision B provides for channel concatenation, in which multiple 1.25 MHz downlink frequency bands and/or multiple 1.25 MHz uplink frequency bands are used together for communications. Such channel concatenation can be used to achieve higher data rates. In addition, Revision B supports “hybrid frequency reuse,” in which different downlink and/or uplink frequency bands are reused among multiple cells or sectors in different ways.
One type of hybrid frequency reuse that has been proposed for Revision B is a K=1/K=3 approach that uses four carrier frequencies that are spread spectrum modulated so as to provide four frequency bands for the downlink (the uplink may be K=1). One of the frequency bands is reused among all of the sectors in a given area, whereas each of three other frequency bands are reused among only certain of the sectors. This K=1/K=3 approach is illustrated in FIGS. 1 and 2.
As shown in FIG. 1, the four frequency bands may be sequential 1.25 MHz CDMA frequency bands, which are identified in FIG. 1 as F1, F2, F3, and F4. The F1 frequency band may be used in all sectors. However, the F2 frequency band may be used in only “alpha” sectors, the F3 frequency band may be used in only “beta” sectors, and the F4 frequency band may be used in only “gamma” sectors. Conventionally, each cell includes one “alpha” sector, one “beta” sector, and one “gamma” sector, which may be arranged as illustrated schematically in FIG. 2. Within each sector, two frequency bands may be concatenated for greater throughput. Thus, F1 and F2 may be concatenated in alpha sectors, F1 and F3 may be concatenated in beta sectors, and F1 and F4 may be concatenated in gamma sectors.
By having one frequency band (F1) common to all of the sectors, soft handoffs between sectors can be facilitated for the reverse link and fast cell site selection can be facilitated for the forward link. However, by also using different carrier frequencies in different sectors, higher data rates and sector throughputs can be supported. In particular, as a mobile station using two frequency bands moves toward the edge of a cell, the mobile station will encounter signals from an adjacent sector in an adjacent cell. The signals from the adjacent sector will include signals in one of same frequency bands used by the mobile station (i.e., from F1, which is common to all of the sectors). However, the mobile station will also be using a frequency band that is not used in the adjacent sector. For example, when a mobile station in a beta sector (using F1 and F3) moves to the cell edge, it will encounter signals from either an alpha sector (using F1 and F2) or a gamma sector (using F1 and F4). In either case, one of the frequency bands used by the mobile station will be non-interfering with the signals from the adjacent sector. This leads to a higher signal-to-noise ratio at the cell edge, which means that higher data rates can be supported at the cell edge. The overall result is that higher average data rates and sector throughputs can be supported throughout the cell.
Although this approach for hybrid frequency reuse can provide advantages, the approach also requires a substantial investment in frequency spectrum. The four sequential 1.25 MHz frequency bands, along with two 625 kHz guard bands, take up a total of 6.25 MHz of frequency spectrum, as illustrated in FIG. 1. This substantial usage of frequency spectrum may limit the applicability of the hybrid frequency reuse approach.
Accordingly, there is a need for methods and system that support hybrid frequency reuse in a more spectrally efficient manner.